Cretaceous

The Cretaceous (IPA: /krɪˈteɪʃəs/ krih-TAY-shəss) is the geological period that lasted from about 143.1 to 66 Ma (million years ago). It is the third and final period of the Mesozoic Era, as well as the longest. The name comes from the Latin word for chalk, which is common in rocks from this time.

The Cretaceous had a warm climate, leading to high eustatic sea levels and many shallow inland seas. These seas were home to many sea creatures that are now gone, like marine reptiles and ammonites. On land, dinosaurs were the main animals.

Many groups of animals and plants that we see today first appeared during the Cretaceous. Early mammals and birds showed up, and flowering plants began to spread and become common. This time was important for the start of many modern life forms.

The Cretaceous ended with a big mass extinction event, where many groups like non-avian dinosaurs and pterosaurs disappeared. Many scientists think this happened because of a huge space rock that hit Earth, creating the Chicxulub crater in the Gulf of Mexico. This event marks the end of the Mesozoic and the start of a new time in Earth's history.

Etymology and history

The Cretaceous period was first described in 1822 by a scientist named Jean d'Omalius d'Halloy. He studied rock layers near Paris and noticed lots of chalk, a type of soft, white rock made from tiny sea creatures. This is why we call it the "Cretaceous," which comes from a Latin word for chalk.

Later scientists split the Cretaceous into different stages to help organize the time period better. One scientist even added extra stages to make the divisions clearer.

Geology

The Cretaceous period was a time when Earth was divided into two main parts called the Early and Late Cretaceous. Scientists also sometimes split it into three smaller parts, but today we mostly use twelve steps to describe changes during this time.

One big change at the end of the Cretaceous was when a large object hit Earth. This caused many plants and animals to disappear, including most dinosaurs. However, some animals like crocodiles and small mammals survived because they could eat things that were already dead or decaying. The rocks and chalk we see today in places like England's white cliffs were formed during this period, showing us how different the world looked millions of years ago.

Paleogeography

During the Cretaceous, the supercontinent Pangaea finished breaking apart into the continents we know today, though they looked quite different then. As the Atlantic Ocean grew wider, big mountain ranges formed along the edge of what is now North America.

Gondwana, a large landmass that included South America, Antarctica, and Australia, also split apart during this time. This created new oceans, including the South Atlantic and Indian Oceans. These changes caused sea levels to rise around the world. In North America, a huge inland sea called the Western Interior Seaway split the land into two parts for much of this period.

The Cretaceous period is well-known for the chalk that formed during this time. Lots of tiny sea creatures made chalk, which now makes up many rock layers we see today. Important rock formations from this time can be found in places like Kansas, Europe, China, and India.

Climate

The Cretaceous period had three main climate phases. It started with a warm and dry time, then shifted to a warm and wet period, and finally ended with cooler and drier conditions.

The climate changed because of Earth’s natural cycles and the movement of ocean currents. During some parts of the Cretaceous, the planet was very warm, with high temperatures even at the poles. This warmth allowed tropical plants and animals to live in places that are now very cold, like near the Arctic and Antarctic. The warm climate also caused big storms and wildfires, which affected forests and plants.

Flora

During the time of the dinosaurs, the world’s plants looked quite different from today. Before flowering plants appeared, the land was mostly covered with plants like conifers and cycads. Flowering plants, also called angiosperms, began to show up around 134 million years ago, with the first signs being tiny pollen grains found in places like Israel and Italy.

These new plants spread quickly during the Cretaceous period. By the end of this time, flowering plants had become the most common plants on land, overtaking groups like conifers. We also see the first signs of grasses and large flowering trees during this period. Even ferns, which are still around today, started to become more diverse as well.

Terrestrial fauna

During the Cretaceous period, many interesting animals lived on land. Small mammals, such as the cimolodont multituberculates, were common and sometimes even more numerous than the famous dinosaurs. Though true marsupials and placentals did not appear until the very end of this time, other types of early mammals were beginning to diversify, filling many roles like hunters, swimmers, and plant eaters.

The biggest land animals were archosaurian reptiles, especially dinosaurs, who were more varied than ever. Early versions of today’s birds were also around, spreading across all continents, even cold places far from the equator. Pterosaurs, flying reptiles, were common early on but became rarer later, with only a few special kinds surviving by the end. Important fossil sites, like the Liaoning area in China, show us well-preserved remains of small dinosaurs, birds, and mammals from the Early Cretaceous, including dinosaurs with hair-like feathers.

Insects also became more diverse during this time. The oldest known ants, termites, and early forms of butterflies and moths appeared. Some notable animals from the Cretaceous include:

• Tyrannosaurus rex, one of the largest land predators of all time, lived during the Late Cretaceous
• Up to 2 m long and 0.5 m high at the hip, Velociraptor was feathered and roamed the Late Cretaceous
• Triceratops, one of the most recognizable genera of the Cretaceous
• The azhdarchid Quetzalcoatlus, one of the largest animals to ever fly, lived during the Late Cretaceous
• Confuciusornis, a genus of crow-sized birds from the Early Cretaceous
• Ichthyornis was a toothed, seabird-like ornithuran from the Late Cretaceous

Marine fauna

During the time of the Cretaceous, many interesting animals lived in the oceans. You could find rays, modern sharks, and teleosts swimming around. Marine reptiles such as ichthyosaurs, plesiosaurs, and mosasaurs also lived in these seas. Sea turtles, including the leatherback sea turtle, were around too. There were even special flightless birds called Hesperornithiformes that dove into the water to hunt.

Other sea creatures included Baculites, a type of ammonite with a straight shell, and reef-building rudist clams. Inoceramids, a kind of bivalve, were common and helped scientists understand changes in the environment. Predatory snails with drilling habits were also widespread. Tiny organisms called globotruncanid foraminifera and echinoderms like sea urchins and starfish thrived. Diatoms, which are a type of algae with glass-like shells, first appeared in the oceans during this time.